The present invention relates to a method and a reactor for in-line production of calcium carbonate (PCC) in connection with the production process of a fibrous web. The invention especially relates to in-line production of PCC into a suspension to be used in the production of the fibrous web, especially preferably directly into the flow of fibrous pulp, one of its partial pulp flows or a filtrate flow used in the production of fibrous pulp.
Calcium carbonate is commonly used in papermaking processes as both filler and coating material due to, among others, the high brightness and low cost of carbonate. Calcium carbonate may be produced by grinding from chalk, marble or limestone, which is then called ground calcium carbonate (abbreviated GCC). Another method of producing calcium carbonate is the chemical method, in which e.g. carbonate ions, formed when the calcium ions, the other constituent of calcium hydroxide, and carbon dioxide are dissolved in water, are allowed to react, whereby the formed calcium carbonate is precipitated from the solution as crystals the shape of which depends on e.g. the reaction conditions. The end product of this production method is called PCC, which is an abbreviation of the words precipitated calcium carbonate. This invention concentrates on the production of PCC and its use especially as a filler of paper.
Traditionally, the production of PCC has taken place separate from the actual papermaking. So far, PCC has been produced either at a dedicated plant located near the paper mill, from which the PCC slurry is directed by pumping along pipelines to paper production, or at a corresponding plant from which the PCC is transported by tank trucks to paper mills located farther away. PCC produced by this method requires the use of retention materials in papermaking in order to have the PCC fastened to the fibers, regardless of whether the fibers are produced chemically or mechanically. The use of retention materials naturally causes additional costs to papermaking in the form of acquiring the chemical itself and as precipitation or recyclability problems possibly caused by the chemical. The traditional method of producing PCC briefly described above brings about problems in addition to the problems relating to the use of retention materials. Tank transportation of PCC to the paper mill from the production site causes transport costs and requires the use of dispersing agents and biocides. The use of the additives affects the properties of PCC while still increasing the acquiring and processing costs.
Building a separate PCC plant in connection with the mill is an expensive investment and the operation thereof requires a workforce of several persons 24 hours a day. A PCC plant according to prior art also consumes large amounts of fresh water and energy.
Thus, lately there have been numerous suggestions for producing PCC directly at the paper mill for reducing the production costs of paper, whereby at least the transport costs of PCC are eliminated from the cost structure of paper. It has additionally been noticed that in-line production of PCC in the presence of fiber suspension leads to better fastening of PCC crystals to the fibers, whereby the need for retention materials is at least reduced and in some cases their use may be totally eliminated. In this context in-line production means producing PCC directly to a suspension used in the production of the fibrous web so that PCC or the suspension is not kept in intermediate storage but it is directly used in the production of the fibrous web. Here, suspension broadly means various liquids transporting fibers or fillers from various high-consistency pulp or stock components to different filtrates formed in the production of the fibrous web, such as any filtrate from a fiber recovery filter.
The newest and currently actually the only industrially applicable method of producing PCC is disclosed in patent application WO-A2-2009/103854. This disclosure teaches production of PCC from carbon dioxide and lime milk so that the carbon dioxide and lime milk are mixed very effectively, preferably by using injection mixers, directly into the pulp in the flow pipe transporting the pulp to the headbox of the paper machine. Thereby, due to the efficient mixing, the carbonate ions and the calcium ions are located close to each other and the formation of crystals is very fast. However, test runs relating to the discussed method have shown that in a way typical to crystallization of calcium carbonate, carbonate crystals are also precipitated onto the surface of the flow pipe in addition to fibers and other solid particles of the target suspension. Carbonate is also precipitated on other solid structures, such as the chemical feed apparatuses and various structures of the mixer. Such precipitations are detrimental to papermaking for example in that when released as smaller or larger particles, a carbonate precipitation spoils the end product, causing, e.g. holes and/or spots to the produced paper or disadvantageous changes in the flows of the headbox, reflected as deterioration of the quality of the end product. Another possible disadvantage is the reduction of mixing due to the reduced functionality caused by the precipitation of carbonate in the feed and/or mixing apparatuses of the chemicals.
The precipitation problems of calcium carbonate are, however, previously known per se. Now, however, the problems have been emphasized when using the injection mixers described in, e.g. patent publications EP-B1-1064427, EP-B1-1219344, FI-B-111868, FI-B-115148 and FI-B-116473 for in-line production of PCC as described in the above-mentioned publication WO-A2-2009/103854. The reason for the increase of problems is that as the injection mixers may mix carbon dioxide and lime milk very fast and evenly into the flow, the duration of the whole crystallization reaction of calcium carbonate is very short. Due to this, a large amount of calcium carbonate in crystallization phase is simultaneously near the wall of the flow pipe so that when said chemicals form a solids crystal it is fastened to the wall of the flow pipe, or in a broader sense, any solid structure being in connection with the flow pipe, and not to another solid material, such as a fiber or a filler particle. Previously, carbon dioxide and lime milk were fed with less powerful mixers, whereby it took the chemicals tens of seconds, sometimes even minutes, to react with another, whereby the carbonate precipitations formed on the inside surface of the flow pipe were distributed on an essentially longer distance of the flow pipe. In other words, while previously precipitations were distributed along the entire length of the short circulation of the paper machine after the introduction point, often to a length of tens of meters, now the precipitations in many cases cover the surface of the flow pipe at a distance of a few meters or even less, measured from the introduction of carbon dioxide and lime milk. In more detail, accumulation of precipitations on the surface of the flow pipe starts at the introduction point of the latter introduced chemical and in practice it ends where at least one chemical has been used up in the crystallization reaction. Because it may be supposed that in the case of both traditional mixing and in mixing using injection mixer essentially the same amount of calcium carbonate is precipitated on the surface of the flow pipe, it is probable that the precipitation layer formed when using injection mixers may in the same period of time be considerably thicker, even many times thicker, than in the traditional mixing method. Simultaneously the risk of the precipitations being broken up and released as fragments to the flow increases and the occurrence rate of problems caused by the fragments may even increase.